1. Field of Invention
The present invention relates generally to power supplies for electronic devices and, more particularly, to electronic devices with integrated power supplies.
2. Discussion of Related Art
Compact “table top” radios and other audio devices (e.g., CD and/or MP3 players, clock radios, etc.) have become a popular choice for people looking for something simpler and smaller than a “boombox” or component-style stereo system It is to be appreciated that, as used herein, a component-style stereo system is one in which external speakers are used.
Some conventional table top audio devices have internal linear power supplies. The basic operating theory of a linear power supply is as follows: The incoming AC (alternating current) voltage is stepped down to a lower AC voltage (e.g., 120 Vac is stepped down to 24 Vac) using a 60 Hz transformer. The AC voltage is then rectified through a full-wave bridge rectifier, usually with a high-current, low-voltage bridge. A filter capacitor is used to maintain a constant dc level with minimum ripple. Linear power supplies offer the advantages of being well known, relatively noise-free and reasonably reliable. They are also generally easy to design and fairly inexpensive to manufacture. However, because of the large 60 Hz transformers required, linear power supplies are generally heavy and rather large. In addition, because a linear voltage regulator operates in its linear region, and all the output current must pass through it, large heat sinks are generally required to dissipate energy loss.
To avoid these disadvantages associated with linear power supplies, other conventional table top audio devices use external switching power supplies which are typically located on the power cord. These external power supplies are referred to as “in-line” power supplies and may include boxes at the plug end of a power cable that are typically referred to as “wall warts.” Switching power supplies operate by rectifying and filtering the incoming AC voltage to obtain a high-voltage DC signal. A low-current, high-voltage bridge rectifier (that may not require a heat sink) can be used, as opposed to the linear bridge used in linear power supplies. The DC voltage is then converted to high frequency AC (typically about 10 kHz to 1 MHz) using electronic switches. The voltage of the high-frequency AC is converted using a high frequency transformer. A benefit of increasing the frequency of the AC signal is that higher frequencies require smaller transformers. At the output of the transformer, the AC is full-wave rectified to an output DC voltage. To keep output DC voltage constant, a sophisticated feedback controller, usually implemented as an integrated circuit chip, is used.
Although switching power supplies are generally smaller and lighter than equivalent linear power supplies, due to the ability to use a smaller transformer, they typically have higher production costs and the use of high frequency switching can result in undesirable noise radiated from the power supply and conducted on the inputs and outputs of the power supply. As a result of this radiated and conducted noise associated with switching power supplies, they typically can only be used as external power sources for audio devices. Of particular concern with the use of switching power supplies in table top radios is that higher frequency harmonics of the power supply switching frequency may generate noise on the signal received by the AM and FM receiver circuitry in the radios. This concern is even greater for radios that utilize the input power line as an antenna.